Photographic image display materials exist in two forms
in the first case, the image is visible against a background layer attached to the photographic material, by coating or lamination, before image formation, development and fixing, as in conventional photographic paper, but sometime added after the image is developed
in the second case the material does not comprise such a background and the image is visible either using transmitted light through the variably transparent layers, or using reflected light off an external background. To this category belong most photographic materials, including negatives, slides, transparencies, movie capture film, movie projection film, microfilm and all kinds of intermediates.
The first patent claiming the use of miniaturized photographic images for security purposes appears to be U.S. Pat. No. 4,243,734, and the method has been extensively applied commercially since. However, some applications are impossible, or require extremely laborious processes, with either of the described types of photographic materials. For example, images on a film cannot be viewed unless they are placed on a sufficiently light-coloured background to provide contrast to the dark areas, or using a back-lit transmission viewing device, since the light areas correspond to light being transmitted unchanged. Several applications of microfilm “microdots”, containing marking codes for security purposes, suffer from severe limitations due to this constraint.
The use of microscopic text or codes, not readable by the naked eye, for security purposes is certainly old, but it also is certainly not becoming obsolete. Microscopic text is part of the security features of banknotes, secure documents, passports, credit cards and other IDs. Printing methods that do not directly rely on photographic imaging can only achieve, until now, image sizes as small as 400 microns (from the specifications of the security printer Zebra P640i). Compare that with an image size achievable on a microfilm of less than 30 microns. Clearly, the use of microfilm as the medium for micro-text marking would be very advantageous, if the limitations mentioned above could be overcome.
Lamination of a white or light-coloured background to a microfilm has been proposed as a way to overcome the limitations of clear microfilm (GB2410208B). However, the photographic material is subjected to thermal and mechanical stresses by the lamination process. The resulting structure has a point of weakness where the laminate is attached to the photographic material, and is not suitable for all liquid environments.